The recent FDA approval of several antitumor agents whose primary mode of action is the inhibition of VEGF receptor-2 signaling and the disruption of tumor angiogenesis (e.g. sunitinib, sorafenib) has had a profound impact on the management of patients with metastatic renal cell carcinoma (RCC). However, despite the ability of these agents to prolong PFS and in some instances, to induce partial tumor regression, the majority of RCC patients develop resistance to these agents within 6-12 months. In previous murine xenograft studies using 786-0 and A498 cell lines we have established that resistance is likely related to angiogenic escape. cDNA microarray and westerns with from xenografts of these RCC cell lines and short-term cultures (STCs) from freshly harvested human RCC identified several genes and proteins whose expression is altered with the development of resistance to sunitinib/sorafenib. These gene expression studies provide several candidate proteins/pathways that, based on their known role in tumor progression and/or angiogenesis, are likely to contribute to the development of resistance. We now propose to extend these studies to include: 1) validation of our murine tissue, blood and imaging findings in patients with RCC undergoing sunitinib therapy; 2) assessment of the ability of pharmacologic inhibitors of these proteins/pathways to either delay or prevent resistance to sunitinib in subcutaneously implanted xenografts involving 786.0 and A498; 3) assessment of promising combinations in subcutaneous and orthotopic xenografts using STCs and perfusion imaging. These studies will lay the groundwork for several clinical trials with sunitinib in combination with other agents selected on the basis of their ability to block one or more critical signaling pathway that might contribute to sunitinib resistance. Endpoints will include changes in tumor perfusion by MRI and serial cytokine levels, and delay in disease progression. Collectively, these studies will elucidate the mechanism by which RCC develops resistance to VEGFR blockade and identify treatment strategies that might circumvent this problem.